1. Field of the Invention
The present invention relates to an optical functioning element which is applicable to optical devices such as a two-dimensional optical image processor, an optical calculation system, a photosensitive IC, etc. and comprises an optical element or combination of a plurality of optical elements, each element being formed on a substrate which is transparent with respect to the wavelength of light emitted from gallium arsenide (GaAs) or aluminum gallium arsenide (AlGaAs) and to the wavelength to which GaAs or AlGaAs is photosensitive.
2. Description of the Related Art
There have been proposed and developed various types of optical functioning elements which function as logical elements and can be made two-dimensionallized such as an optical bistable circuit, a differential gain circuit, or optical switch.
Most of the optical functioning elements have a structure which enables the element not only to receive an optical input from outside but emit an optical output to outside as well. A first type optical functioning element is one which comprises a substrate and which outputs the optical signal from the front surface of the substrate and receives the input signal from outside through the same front surface of the substrate. Another type of optical functioning element is one which comprises a substrate and which outputs the optical signal from the front surface of the substrate and receives the input signal from the rear surface of the substrate or vice versa.
The optical functioning element of the type which receives an input signal from one surface thereof and outputs the signal from the opposite surface thereof is disclosed in IEEE Quantumn Elect. Vol QE-21(9)(1985)1462.
The disclosed element comprises an n-type GaAs substrate on which are stacked in this order an n-type AlGaAs layer, an n-type superlattice layer comprising alternately stacked n-type GaAs films of several ten .ANG.s thick and n-type AlGaAs films of the same thickness, a multiquantum well (MQW) layer comprising alternately stacked GaAs films of a little less than 100.ANG. thick and AlGaAs films of the same thickness, s superlattice layer comprising alternately stacked p-type GaAs films of several ten .ANG.s thick and p-type AlGaAs films of the same thickness, and a p-type AlGaAs layer. The n-type GaAs substrate is etched to form a hole of about 100.ANG. diameter therein to reach the n-type AlGaAs layer to constitute a mesa structure at the position behind and corresponding to the stacked-layer structure mentioned above. Due to this hole, the light of the above mentioned wavelength penetrates and passes through the stacked-layer structure without being blocked by the substrate.
That is, in accordance with the optical functioning element mentioned above, an optical logic function is obtained by vertically passing the light through the element from the front surface to the rear surface or vice versa. For this purpose, the window (hole) is formed through the substrate so as to prevent the light from being absorbed by the substrate of n-type GaAs.
Next, a second type of optical functioning element which receives the input optical signal from the front surface thereof and outputs the signal from the same front surface is disclosed in Appl. Phys. Lett. Vol 52(1988)679.
The disclosed element comprises an SI (Semi-Insulation)-GaAs substrate on which are stacked in this order an n-type GaAs layer, an n-type AlGaAs layer, an n-type GaAs layer, a p-type GaAs layer, a p-type AlGaAs layer, and a p-type GaAs layer. The input light is transmitted to the element from the uppermost layer of the stacked structure and the output light is emitted from the same uppermost layer so as to achieve a memory function or other logic functions.
A third type of optical functioning element which receives the input light from the rear surface thereof and emits the output light from the front surface thereof is disclosed in J. Lightwave Tech. Vol LT-3(6)(1985)1264.
The disclosed element comprises an n-type InP substrate, a light receiving portion formed on the substrate and a light emitting portion formed on the light receiving portion. The light receiving portion is composed of an n-type InP emitter, a p-type InGaAsP gate, an n-type InGaAsP buffer, an n-type InP buffer, and an n-type InGaAsP absorber, stacked on the substrate in this order. The light emitting portion is composed of an n-type InP confining layer, an n-type InGaAsP active layer and a p-type InP confining layer, stacked on the light receiving portion in this order so as to achieve an optical logic function.
In accordance with the above mentioned third structure, the forbidden band width of the n-type InP is wider than that of the InGaAsP layer which generates or absorbs light and the InGaAsP layer's lattice matches with the InP layer's lattice. Therefore, it becomes possible for the light to access the element from either side of the element having the substrate being attached thereto. However, with regard to the GaAs group or the AlGaAs group, the substrate of GaAs has the narrowest forbidden band width of all the materials in the group. Therefore, it is impossible to realize the structure of the third type mentioned above with the use of the GaAs or AlGaAs group materials.
Also, in IOOC '89, Lecture No. 18B2-6 is proposed a fourth type of optical functioning element comprising a GaAs substrate and an InGaAs layer formed on the substrate. The InGaAS layer has a lattice constant which is far different from that of the GaAs substrate and a forbidden band width which is narrower than that of the GaAs substrate. By stacking the InGaAs layer on the GaAs substrate, the substrate becomes transparent with respect to the accessing light having a predetermined wavelength.
In the event that the light emitting portion (layer) or light receiving portion (layer) of the element to which the light accesses is made from GaAs or AlGaAs, the substrate on which the layer is stacked is made from GaAs from the standpoint of lattice matching.
In accordance with the first example of the optical functioning element of the related art mentioned above, a part of the GaAs substrate is removed so that the light is not absorbed by the substrate at this part. The removing process is conducted in such a way that this part of the substrate is etched with the use of an etching solution having different etching rates (effects) with respect to GaAs and AlGaAs, for example an ammonium solution, so that the etching process is stopped at the surface of the AlGaAs layer whereby only the GaAs layer is selectively etched and removed.
It is to be noted that the GaAs substrate has to be at least 70 to 100 .mu.m thick to maintain the strength thereof and to prevent it from being distorted.
Also, the substrate is etched not only in the vertical direction (perpendicular to the substrate surface) but also in the lateral direction thereof with the same etching rate so that a hole is formed in the substrate which hole has a diameter twice as large as the depth thereof. Such a hole hampers the close arrangement of a plurality of elements in a two-dimensional plane to constitute an array device. Therefore, when the array device is to be made, one common hole is formed for a plurality of elements instead of forming a hole for each of respective elements, which reduces the strength of the device and makes it difficult to widely arrange a large number of elements since the device becomes curved and distorted. Especially, when the element has a light emitting function, heat is generated from the element, which impairs the functional reliability of the element since the element itself does not effectively radiate heat. Therefore, a plurality of elements have to be directly mounted on a heat radiator plate made from transparent materials, for example, sapphire, which involves problems such that electrode patterns have to be precisely formed on the radiator plate, that the periphery of the element has to be covered, and that the element has to be accurately positioned with respect to the radiator plate. Therefore, it becomes very difficult to realize a high density two-dimensional array device using the optical functioning elements of the related art mentioned above.
Besides, there are further problems at the time of producing the element such that it becomes necessary to form patterns on both of the upper and lower surfaces of the substrate, and that the n-type AlGaAs layer surface which is exposed by the partial etching of the GaAs substrate mentioned above is roughened by the etching process, which induces the loss of optical power of input and output signals.
The second example of the optical functioning element mentioned above has the structure in which the accessing light is received and emitted from the same surface of the element substrate, which obviates the problems of the first example mentioned above.
However, in accordance with the structure of the second example, it becomes necessary to prepare an optical system or device for separating the input and output light beams since the optical systems or devices for receiving input beams from and for transmitting output beams to are disposed in the same side of the element. This hampers the realization of a small and compact optical system using the elements to constitute a two-dimensional device. Besides, another problem arises that it becomes necessary to accurately align the element with the other optical systems.
Next, with regard to the third example of the optical functioning element of the related art mentioned above, the structure of this example can not be applied to the structure in which the GaAs or AlGaAs group materials are used. This is because light can not penetrate through the substrate since the forbidden band width of the substrate made from GaAs is the narrowest.
Also, in accordance with the fourth example of the optical functioning element of the related art mentioned above, the light emitting layer is made from InGaAs which has a narrower forbidden band width than GaAs. Therefore, the InGaAs layer has to be stacked on the GaAs substrate. However, the lattice structure of the InGaAs layer does not match with that of the GaAs substrate, which causes distortion of the element. Therefore, it becomes difficult to heighten the quality of the stacked-layer structure of the element and easy to generate the dislocation defects. Also, due to the heat generated from the element, the line defects of the structure are generated and transferred within the structure, which degrades the quality of the element and impairs the stable function of the element.